Titanic coming to a halt at sea (5)
I believe the answer is:
avast
'halt at sea' is the definition.
(nautical term for 'stop')
'titanic coming to a' is the wordplay.
'titanic' becomes 'vast' (both can mean very large).
'coming to' says to put letters next to each other.
'vast' after 'a' is 'AVAST'.
(Other definitions for avast that I've seen before include "eleven [CEASE] at sea" , "Stop (nautical)" , ""stop" signal" , "Hold fast! (naut.)" , "order for tar" .)